It is known in the art of optical compensation that the phase retardation of light varies according to wavelength, causing color shift. This wavelength dependence (or dispersion) characteristic of the compensation film may be taken into account when designing an optical device so that color shift is reduced. Wavelength dispersion curves are defined as “normal (or proper)” or “reversed” with respect to the compensation film having positive or negative retardance (or retardation). A compensation film with positive retardance (positive A- or C-plate) may have a normal curve in which the value of phase retardation is increasingly positive toward shorter wavelengths or a reversed curve in which the value of phase retardation is decreasingly positive toward shorter wavelengths. A compensation film with negative retardance (negative A- or C-plate) may have a normal curve in which the value of phase retardation is increasingly negative toward shorter wavelengths or a reversed curve in which the value of phase retardation is decreasingly negative toward shorter wavelengths. Exemplary shapes of these curves are depicted in FIG. 1.
Wave plates are customarily named as follows in accordance with their refractive index profiles:
positive C-plate: nx=ny<nz; negative C-plate: nx=ny>nz 
positive A-plate: nx>ny=nz; negative A-plate: nx<ny=nz 
wherein, nx and ny represent in-plane refractive indices, and nz the thickness refractive index. The above wave plates are uniaxial birefringent plates. A wave plate can also be biaxial birefringent, where nx, ny, and nz are all not equal; it is customarily named as biaxial film.
An A-plate having in-plane retardation (Re) equal to a quarter of the wavelength (λ/4) is called quarter wave plate (QWP). Likewise, an A-plate having Re equal to half of the wavelength (λ/2) is called half wave plate (HWP). An ideal achromatic QWP would be able to retard an incident polarized light by λ/4 at every wavelength. In order to achieve this, the wavelength dispersion of the QWP has to be reversed and satisfies the following equations:Re(450)/Re(550)=0.818 and Re(650)/Re(550)=1.182,wherein Re(450), Re(550), and Re(650) are in-plane retardations at the light wavelengths of 450 nm, 550 nm, and 650 nm respectively. An achromatic (or broadband) wave plate is highly desirable since it can direct the light in the same manner at each wavelength to yield the optimal viewing quality. A common wave plate, however, exhibits a normal dispersion curve, which is not suitable for broadband wave plate application. Thus, there exists a need for a wave plate having reversed wavelength dispersion characteristics with respect to in-plane retardation.
A-plates are commonly used in liquid crystal displays (LCDs) as compensation films to improve the viewing angles. They can also be used in an OLED (organic light emitting diode) display device. For example, a QWP is being used with a linear polarizer to provide a circular polarizer in an OLED device to reduce the ambient light reflected by OLED for improved viewing quality. These applications typically utilize the in-plane retardation provided by the A-plate for in-plane phase-shift compensation. For example, A-plate combining with C-plate is particularly useful in reducing light leakage of the crossed polarizers at oblique viewing angles. The A-plate, however, also exhibits a negative out-of-plane retardation Rth, which is defined as Rth=[nz−(nx+ny)/2]×d with a value of |Re/2| arising from its orientation. This characteristic can be beneficial when a negative Rth is desirable in an optical device. For example, in a vertically aligned (VA) mode LCD, the liquid crystal molecules in the LC cell are aligned in a homeotropic manner, which results in positive retardation. An A-plate, thus, can provide an out-of-plane compensation in addition to in-plane compensation in VA-LCD. In other devices, such as in-plane switch (IPS) mode LCD and OLED display, however, the Rth exhibited in the A-plate is not desirable since it can give rise to phase shift in off-axis light and lead to light leakage. Thus, there exists an additional need in the art to provide a positive in-plane retarder having reduced out-of-plane retardation for improved viewing angle and contrast ratio of the display.
U.S. Pat. No. 7,480,021 discloses an optical film having reversed birefringence dispersion comprising a first component having a normal dispersion and a second component having a reversed dispersion, wherein the two components have the same sign of birefringence.
U.S. Pat. No. 7,948,591 discloses a uniaxial retardation film satisfying the equations of 118 nm≤Rxy(550)≤160 nm, −10 nm≤Ryz(550)≤10 nm, 0.75≤Rxy(450)/Rxy(550)≤0.97, and 1.03≤Rxy(650)/Rxy(550)≤1.25. U.S. Pat. No. 8,139,188 discloses a biaxial retardation film satisfying the equations of 220 nm≤Rxy(550)≤330 nm, 110 nm≤Rxz(550)≤165 nm, 0.75≤Rxy(450)/Rxy(550)≤0.97, and 1.03≤Rxy(650)/Rxy(550)≤1.25. In both patents, no materials that can satisfy the specified equations are disclosed.
US Patent Application No. 2008/0068545 discloses a retardation film comprising a film, which is a film comprising a fumaric ester resin and satisfying nx<ny≤nz, and a film satisfying ny>nx≥nz or ny>nz≥nx. The disclosed film may have a reversed dispersion characteristic.
US Patent Application No. 2012/0003403 discloses a multilayer film comprising (a) a layer (A) comprising cellulose ester having a degree of substitution of hydroxyl groups (DSOH) of 0 to 0.5 and (b) a layer (B) comprising cellulose ester having a DSOH of 0.5 to 1.3, wherein the film has a reversed optical dispersion.